This invention relates generally to devices for the emission of a gas at a constant rate, and, more particularly, to such gas emitting devices used in the production of calibration samples for gas or liquid analyzers. Gas is emitted from such a device through a permeable material, and mixed with a moving fluid medium for use in an analyzer.
This inventor's prior patent, U.S. Pat. No. 3,856,204, represented a significant advance in the field of gas emitting devices. Prior to the invention described and claimed in the patent, gas emitting devices had employed an elongated tube of a permeable, polymeric plastic material to hold the gas under pressure and partially in the liquid phase. At a constant temperature, the vapor pressure of the substance is also constant, and molecules of the substance permeate through the walls of the tube at a constant rate, thereafter to be intermixed or dissolved in a moving fluid medium stream surrounding the tube. However, as discussed in the aforementioned patent, tube devices of this type suffered from major disadvantages. In the invention disclosed in the prior patent, the substance to be emitted is held in a sealed vessel having a passage filled with a permeable polymeric plastic material, through which the substance can permeate and be emitted outside the vessel. The permeable material used in the device provides for use of a much higher permeation rate, and the device has a much lower sensitivity to temperature variations, is comparison with the prior devices.
Although the device disclosed in the inventor's prior patent represented a significant advance in the field, the device still falls short of perfection in some areas of application. Although the device has relatively low sensitivity to temperature variations, it is nevertheless more sensitive to temperature variations when the substance to be emitted is stored in the liquid phase. When the substance is stored in the gaseous phase, an extremely favorable temperature characteristic is obtained. However, much smaller quantities of the substance can be held in the gaseous phase than in the liquid phase. When relatively high permeation rates are required, such as 20,000 to 50,000 nanograms (ng) per minute or higher, an impractically large gaseous phase permeation device is needed, or its useful life is extremely limited. Liquid phase devices, on the other hand, permit the use of relatively high permeation rates and long useful lives, but suffer from the significant drawback of a relatively poor temperature characteristic. Another difficulty with liquid phase devices is that the liquid substance is sometimes detrimental to the permeable material, greatly reducing the life of the device. Furthermore, the permeable material can be dislodged from the passage and the substance stored under pressure can be released in an explosive and sometimes dangerous manner.
It will be apparent from the foregoing that there is still a significant need for improvement in gas emitting devices of this type. What is particularly needed is a device retaining the advantages of the one disclosed in the inventor's prior patent, but avoiding its inherent disadvantages. The present invention fulfills this need.